1. Field of the Invention
The invention is generally directed to assemblies for transmission lines that include a heat sensitive dielectric, and is more particularly directed to methods and apparatuses for coupling transmission lines to circuit boards.
2. Description of the Related Art
Various schemes for coupling transmission lines to circuit board assemblies are known in the prior art. Many of such prior art schemes fall into one of three basic categories:
1. In well known connector type schemes, various connectors for use at high frequency (defined herein substantially within a range from approximately one Mega-Hertz to approximately five Giga-Hertz, so as to also include ultra-high frequency), for example BNC, SMA, SMB, TNC, MCX, MMCX, N, F, UHF and Mini UHF type connectors, well known in the art, are employed. For the connector scheme, the transmission line is coupled to half of the connector system, another half of the connector system is soldered to the printed circuit board, and then both halves of the connector are mated to provide electrical connection.
2. In a manual direct soldering scheme, the transmission line is soldered directly (usually by hand) to plated areas (pads or vias) in the printed circuit board.
3. In an automated direct soldering scheme, conductors of the transmission line and their corresponding mating pads on the printed circuit board are each tinned or provided with a supply of solder, usually in the form of solder paste. The conductors of the transmission line are held in place on their mating pads with fixturing, while a heat source (anvil, hot air, or LASER) is used to reflow the joints.
Advantages of the first scheme are primarily limited to those situations where multiple cycles of connecting and disconnecting the transmission line from the circuit board are needed. However, in many cases only the connection is needed. Disconnection isn""t needed or desired. Disadvantages of the connector scheme include the complexity, unreliability, ohmic signal loss, cost and size of the mating connector. For extremely small printed circuit board assemblies, large connectors are particularly undesirable. For example, with extremely small circuit boards (such as those on the order of approximately a half an inch square) used in oscilloscope probe assemblies, the size of a connector may rival or exceed that of the circuit board.
The second scheme of manual direct soldering provides advantageous low ohmic signal loss. The disadvantages of this scheme include the time consuming, expensive manual labor, and further include undesirable variability of hand soldered connections. Also, it is usually necessary, when working with a multistranded conductor of a transmission line, for example an outer braid conductor of a coaxial cable, to prepare an extremity of the multistranded conductor with twisting and/or tinning to keep the individual strands from unraveling. These processes are usually done by hand, and suffer from the same variability and time consuming aspects as any other hand operation. Additionally, care must be used in hand soldering when the transmission line includes heat sensitive dielectric.
The third scheme is an attempt to automate the manual direct soldering of the second scheme. Process variables and difficulties in holding conductors of the transmission lines in place make this scheme only slightly better than hand soldering. When the transmission line includes heat sensitive dielectric, care must be used with the heat source (anvil, hot air, or LASER) used to reflow the joints.
The heat sensitive dielectric, as defined herein, is one that may be substantially deformed or otherwise damaged if exposed to excessive heat during the soldering. Such deformation or other damage caused by such excessive heat may interfere with insulation, for example, the dielectric insulating a first conductor of the transmission line from an additional conductor of the transmission line, resulting in a short circuit. Alternatively, such deformation or damage of the heat sensitive dielectric may introduce a lesser electrical discontinuity for high frequency signals, that still substantially interferes with signal integrity.
Examples of such heat sensitive dielectrics are included in the following table. In the table, following each name is a respective approximate maximum service temperature. The approximate maximum service temperature is a recommended temperature that the particular dielectric material can withstand. At a substantially higher temperature, deformation, melting, or even scorching of the dielectric may take place. Of course, there is variation in these temperatures based on factors such as particular formulations. However the temperatures summarized in the table are generally considered to be fairly representative in the literature, such as plastics encyclopedias.
It should be particularly noted that all temperatures for the examples of heat sensitive dielectrics in the table are well below that a melting point of a typical solder (for example a eutectic solder having a composition of 63% Tin (Sn) and 37% Lead (Pb) has a melting point of three-hundred and sixty-one degrees Fahrenheit (361 F.).) Accordingly, it should be understood that excessive heat of soldering poses a real and substantial threat to integrity of heat sensitive dielectrics.
While the schemes of the prior art discussed previously herein provide some advantages, some challenges still remain, particularly for transmission lines and circuit boards used in extremely small probing arrangements for oscilloscope systems, and the like. For such systems, there is a need to make finished probe tip assemblies which are small enough that their weight does not bend or break extremely delicate leads of fine-pitch (and extra-fine-pitch) surface mount integrated circuit packages. Also, their size must be reduced so that probe assemblies can be placed side-by-side in little space to probe multiple closely spaced circuits. The most common scheme for terminating transmission lines in this type of assembly is hand soldering. Trouble with inconsistencies in hand soldering led to a search for other termination schemes. After considerable research, it was found that conventional hand soldering had the best probability of success, despite its shortcomings. So-called xe2x80x9cmicrominiaturexe2x80x9d connectors were much too large, while hot-anvil or LASER reflow systems were extremely process-dependent and would rely on manual alignment of transmission lines, with little improvement over hand soldering in terms of consistency.
Accordingly, there is a need for improved systems and methods which address these and other shortcomings of the prior art.
Briefly described, the present invention generally relates to devices, systems and methods for coupling transmission lines to circuit boards. As the present invention may be construed as providing a method for coupling a transmission line to a circuit board, with the transmission line including a heat sensitive dielectric and at least a first conductor, a preferred embodiment of such a method includes the steps of: providing a circuit board having a first side and an opposing second side; dispensing solder upon the first side of the circuit board; providing a first coupler having a conductive lead portion and a ductile conductive tube portion extending from the conductive lead portion; engaging the conductive lead portion with the solder dispensed upon the first side of the circuit board; heating the circuit board, solder and the conductive lead portion such that the solder reflows, thereby securing the first coupler to the first side of the circuit board; inserting an extremity of the first conductor into the ductile conductive tube portion; and crimping the ductile conductive tube portion so as to provide crimped retention of the first conductor therein.
Other embodiments of the present invention may be construed as providing circuit boards. In a preferred embodiment, the circuit board includes a substrate with a first side and an opposing second side and solder applied in a land pattern to the first side of the substrate. The circuit board also includes a first coupler engaging the first side of the circuit board that is secured thereto by the solder. The first coupler incorporates a first conductive lead portion and a first ductile conductive tube portion extending from the first conductive lead portion. Additionally, a first transmission line is provided which includes a first conductor and heat sensitive dielectric engaged about at least a portion thereof, with an extremity of the first conductor being engaged in crimped retention within the first ductile conductive tube portion.
An alternative embodiment of the circuit board includes substrate with a first side and an opposing second side and solder applied in a land pattern to the first side of the substrate. A first transmission line is provided which includes a first conductor and heat sensitive dielectric engaged about at least a portion thereof. Additionally, means for engaging an extremity of the first conductor in crimped retention is provided so that the transmission line may be coupled to the circuit board.
Other embodiments of the present invention may be construed as providing systems for coupling a transmission line to a circuit board. In a preferred embodiment, the system includes a circuit board, a solder dispenser configured to dispense solder upon a first side of the circuit board, and a first coupler. Automated pick-and-place equipment is configured to extract the first coupler from a presentation and engage the conductive lead portion of the first coupler with the solder dispensed upon the circuit board. A reflow heater assembly is provided, which is configured for receiving the circuit board, solder and the conductive lead portion from the automated pick-and-place equipment and heating the circuit board, solder and the conductive lead portion so that the solder paste reflows, thereby securing the first coupler to the first side of the circuit board. Additionally, crimping equipment is provided for receiving the circuit board with the first conductor inserted into the ductile conductive tube portion and for crimping the ductile conductive tube portion so as to provide crimped retention of the first conductor therein.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such features and advantages be included herein within the scope of the present invention, as defined in the appended claims.